1. Field of the Invention
The present invention relates in general to variable valve mechanisms of an internal combustion engine, and more particularly to the variable valve mechanisms of a type that induces an open/close operation of engine valves (viz., intake and/or exhaust valves) while varying a valve lift degree and an operation angle of the engine valves in accordance with an operation condition of the engine. More specifically, the present invention is concerned with an actuation mechanism for actuating the variable valve mechanism.
2. Description of the Related Art
Hitherto, various variable valve mechanisms have been proposed and put into practical use in the field of automotive internal combustion engines. One of such mechanisms is the mechanism disclosed in Japanese Laid-open Patent Application (Tokkai) 2004-76621.
That is, the variable valve mechanism of the Laid-open Patent Application generally comprises a drive shaft that is driven by a crankshaft of the engine, two united swing cams for each cylinder that induce an open/close operation of respective intake valves when swung, a drive cam for each cylinder that is tightly disposed on the drive shaft, a transmission mechanism for each cylinder that includes a rocker arm and is arranged between the drive cam and one of the swing cams to transmit movement of the drive cam to the swing cam while converting a rotary motion of the drive cam to a swing motion of the swing cam (or swing cams), a control mechanism that includes an eccentric control cam operatively received in a circular opening of the rocker arm and a control shaft for controlling a rotary motion of the eccentric control cam, and an actuation mechanism that controls a rotary motion of the control shaft in accordance with an operation condition of the engine.
The actuation mechanism comprises an electric motor, a ball screw shaft that is actuated by the electric motor to rotate in normal and reversed directions, a ball nut member that is operatively engaged with the ball screw shaft and axially movable along and on the ball screw shaft when the ball screw shaft is turned, a link member that has a forked end pivotally connected through respective pins to diametrically opposed parts of the ball nut member, and a connecting arm that has one end secured to one end of the control shaft and the other end pivotally connected to the other end of the link member.
The actuation mechanism further comprises first and second stopper pins that restrict the maximum rotary movement of the control shaft in both normal and reversed directions, and first and second coil springs that function to bias, through the ball nut member, to turn the control shaft in a direction to increase or decease the valve lift degree just before the control shaft is stopped by the first and second stopper pins upon stopping of the engine.
Because of employment of the first and second coil springs, even when, with the ball nut member assuming the frontmost or rearmost end position on the ball screw shaft, the electric motor fails to operate, the control shaft is assuredly turned in one direction to and held at a certain angular position that assures a certain valve lift degree of the intake valves. The valve lift degree thus assured facilitates engine starting even in a cold condition.